The most widely used industrial hydrodesulfurization (HDS) catalysts are derived from alumina-supported oxides of Mo (or W) and Co (or Ni) which are sulfided for use. See O. Weisser et al., Sulphide Catalysts, Their Properties and Applications, Pergamon Press, Oxford, U.K. (1973). HDS research has focused on the elucidation of catalyst structure and composition and the nature of the active sites. For example, see H. Topsoe et al., Catal. Rev.-Sci. Eng., 26, 395 (1984). Several years ago, a new class of materials referred to as Chevrel phases or "reduced" molybdenum sulfides were identified as potential HDS catalysts. See G. L. Schrader et al., in Advances in Hydrotreating Catalysts, Vol. 50, M. L. Ocelli et al., eds., Elsevier, Amsterdam (1989) at page 41. Chevrel phase materials are ternary molybdenum chalcogenides of the general formula M.sub.x Mo.sub.6 Z.sub.8, wherein M can be over forty different elements and Z is S, Se or Te. See R. Chevrel et al., Topics in Current Physics, 34, 25 (1982). Bulk Chevrel phase catalysts were shown to have comparable or higher thiophene and benzothiophene HDS activities compared to conventional catalysts (normalized to the surface area of bulk samples). In addition, Chevrel phases were shown to favor desulfurization over hydrogenation (HYD), making them potentially more selective catalysts.
Current methods for preparing highly pure bulk Chevrel phase materials rely primarily on solid state preparation techniques using stoichiometric amounts of metal sulfides in combination with either MoS.sub.2 or Mo and S. Powders of these components are thoroughly ground together and heated in fused-quartz tubes at temperatures between 1000.degree. C. and 1200.degree. C. for periods of as long as several days (J. W. Lynn et al., Phys. Rev. B, 24, 3817 (1981)).
Such techniques are not easily adaptable for preparation of supported Chevrel phases. Supported catalysts are useful for industrial applications in which a high surface area support is needed for catalyst dispersion. However, it is also possible that catalytic properties may be altered by the process of binding the catalyst to the support. For HDS catalysts, different exposures of edge or corner sites may occur which would probably alter the number of active sites. See H. Topsoe et al., Catal. Rev.-Sci. Eng., 26, 395 (1984); O. P. Bahl et al., Proc. Roy. Soc. London, Ser. A306, 53 (1968). The catalyst also may react with the support and subsequently lose activity or selectivity.
Reactive sputtering is a potentially useful method for depositing Chevrel phases on supports. Previous attempts to sputter Chevrel phases have been motivated by the superconducting properties of these materials. See P. Przyslupski, in Ternary Superconductors, Shency, Dunlop, Fradin, eds., Elsevier (1981) at page 125. PbMo.sub.6 S.sub.8, BaMo.sub.6 S.sub.8, AgMo.sub.6 S.sub.8, Sn.sub.1.2 Mo.sub.6.4 S.sub.8, Cu.sub.3.5 Mo.sub.6 S.sub.8, Ho.sub.x Mo.sub.6 S.sub.8, and LaMo.sub.6 S.sub.8 have been produced by sputtering techniques. Three general approaches have been used to obtain stoichiometric films. First, composite targets have been used in which the target is a hard, pressed tablet, fabricated from a preselected mixture of the powders of the elements corresponding to the desired composition. Second, sintered targets of Chevrel phases have been used. Both of these approaches require post-annealing of the deposited (amorphous) material at elevated temperatures in order to form single phase films. The third sputtering technique which has been employed to produce a restricted number of non-lead Chevrel phases uses reactive sputtering of multiple targets in an Ar/H.sub.2 S atmosphere; deposition occurs directly on the heated substrate. This approach eliminates the need for composite or sintered targets and also offers advantages of one-step preparation. See G. B. Hertel et al., J. Appl. Phys., 61, 4829 (1987).
However, difficulties are encountered in using sputtering to deposit lead Chevrel phases. The relative concentration of three constituents has to be controlled with a very high degree of accuracy because of the narrow phase range of the lead Chevrel phase. See J. M. Tarascon et al., J. Sol. State Chem., 54, 204 (1984). Moreover, Pb (m.p. 327.5.degree. C.) and S (m.p.=115.2.degree. C.) are both highly volatile at the temperature used for formation of the lead Chevrel phase (about 1050.degree. C.). Therefore, only Ag.sub.x Mo.sub.6 S.sub.8 and Cu.sub.x Mo.sub.6 S.sub.8 films have been successfully prepared by sputtering in situ on hot substrates (850.degree. C.). See G. Hertel et al., J. Apl. Phys., 61, 4829 (1987) and P. Przyslupski et al., Solid State Com., 28, 869 (1978).
In 1978, P. Przyslupski et al., in Solid State Com., 28, 869 (1978) reported the successful sputter deposition of Pb Chevrel phase thin films. A pressed composite target formed from a mixture of MoS.sub.2 and PbS powders was used. These materials were sputtered onto sapphire substrates at ambient temperature. Deposited films were amorphous, and annealing in sealed quartz tubes at 850.degree. C. to 1050.degree. C. was used to produce crystalline films of PbMo.sub.6 S.sub.8.
Targets consisting of sintered lead Chevrel phases have also been used, for example, by H. Adrian et al., Physica, 1078, 647 (1981). Again, annealing the as-deposited films was required. These targets generally have limited versatility in varying the stoichiometry of the deposited material.
A more modular target was used by C. K. Banks et al., J. Solid State Chem., 15, 271 (1975). MoS.sub.2 was overlaid with wedges or sheets of PbS, and in this case, the composition of the film could be varied by changing the dimensions of the metal sulfide sheets. However, because some components of the target were not good conductors, RF-sputtering had to be employed. The variable composition of these targets can also lead to differences in the sputtering rate of each component, and this film homogeneity.
Therefore, a continuing need exists for a method to reproducibly form homogeneous coatings of a variety of lead Chevrel phases on various substrates at acceptable deposition rates.